Seismic data collected in the marine environment are often contaminated by multiple reflections. The desired data come from once-reflected seismic waves, where the reflection occurs at a subsurface (water bottom or below) reflector. Waves that reflect one or more additional times off the air-water interface or any other reflector (i.e., “multiples”) confuse the data interpretation and thus constitute unwanted noise when detected and recorded at a survey receiver along with the once-reflected data. Removal of multiples is often incomplete because the algorithm employed to remove the multiples is ineffective. As a result, the seismic image generated from the data is degraded; and incompletely removed (i.e., “residual”) multiple energy may be confused with the true seismic image.
The present invention concerns the processing and analysis of OBS data for the identification and suppression of multiples. Of specific concern is the identification and suppression of a common type of multiple that arises from reverberation of seismic energy within the water column. FIG. 1 illustrates an example of a common type of such a multiple reflection 19. In FIG. 1, a seismic source 11 emits an acoustic wave indicated by ray 13 which is reflected off some interface 16 located in the subsurface below the sea floor 17. The reflected wave is detected by hydrophone 14 and by geophone 15 receivers located on the sea floor. This is called the primary reflection, and its detection by the receivers constitutes the desired gathering of information. However, the upcoming wave 13 will continue upward through the water column 18 to be reflected by the air water interface 12 back down toward the receivers and recorded at a later time than the primary reflection. The time delay between the recording of the primary and the multiple is equal to the time taken for the seismic energy to travel through the water column. Thus, multiple reflections interfere with later arriving primary reflections generated by deeper subsurface reflectors. Diminishing amounts of this multiple energy will continue to reverberate 19 in the water column between the sea floor and surface, and will be recorded as unwanted noise appearing on top of and tending to mask the desired primary reflection detections.
In ocean bottom seismic surveys, instead of being towed along with the seismic source by a vessel, the survey receivers are placed on the ocean bottom in stationary locations. As used herein, the term ocean bottom seismic, or OBS, includes the common practice of installing receivers in cables strung along the ocean floor (called ocean bottom cable or “OBC”) as well as receivers installed as isolated nodes on the sea floor. OBS surveys are sometimes two component (“2C”) meaning that they contain hydrophone and vertical geophone receivers. Sometimes they are four component (“4C”) meaning that they contain hydrophone and three (vertical and two horizontal) geophone receivers. Accordingly, as used herein, “2C” will be understood not to exclude the possibility of additional geophones at a receiver location, measuring additional components.
One of the main advantages of two component (“2C”) OBS seismic is an ability to record a propagating wave using both vertical geophone (“Z-component”) and hydrophone (“P-component”). The hydrophone measures the pressure in the water column, a scalar property which, therefore, is not sensitive to direction of the wave propagation. The vertical geophone measures the vertical component of the particle motion (for example, velocity or acceleration) of the seafloor (a vector property) and is, therefore, sensitive to the propagation direction. Alternatively, MEMS receivers (Micro-Electro Mechanical Systems) may be used instead of geophones to supply the vector signal. The directional sensitivity of the OBS system allows discrimination of upgoing (including the signal component) and downgoing (including water column multiples) seismic events. In particular, the hydrophone and geophone may be arranged such that the upgoing energy is measured with the same polarity on the two detectors, whereas downgoing energy is measured with opposite polarity. This effect provides the basis for attenuating water column reverberations using OBS hydrophone and geophone combinations, so-called “PZ-combinations” (e.g., U.S. Pat. No. 6,678,207 to Duren). General industry practice is to perform this combination of hydrophone and geophone data before stacking of the data (i.e., early in the processing flow). For instance, Corrigan (U.S. Pat. No. 5,696,734) teaches application of the combination using common-receiver gathers of seismic data.
However, one of the major limitations of such pre-stack combinations of hydrophone and geophone data arises from the fact that pre-stack data have a low signal-to-noise ratio. Thus, the effectiveness of combination techniques, that require signal estimation or analysis, may be reduced. A major purpose of stacking seismic traces from different records is to increase signal relative to noise by cancellation of random noise.
Many 2C OBS combination methods require an estimate of the water-bottom reflection coefficient. Because this information is difficult and costly to acquire, and required as a function of offset for pre-stack combination, the effectiveness of many 2C OBS combination methods may be compromised. However, in Duren's method (U.S. Pat. No. 6,678,207), the water-bottom reflection coefficient is not required; it is internally estimated.
Another limitation of most 2C OBS combinations is the requirement of proper scaling of geophone and hydrophone signals and matching of the P and Z wavelets. Those skilled in the art will recognize this as a difficult part of the process.
A common result of the imperfect combination of OBS hydrophone and geophone data arising from these limitations is a degraded seismic image and the presence of residual multiple energy on the combined PZ image. This energy may be confused with true seismic image.
Furthermore, the identification of multiples on images by those skilled in the art is not straightforward. A common method for identifying multiples relies primarily on visual analysis. Strong multiples often have a different character (amplitude, orientation, periodicity) than true seismic energy. However, weak residual multiples are often very similar in character to primary energy. As described above, multiples are related to their generating primary event by a time approximately equal to the two-way travel time to the water bottom. A rudimentary way of identifying multiples is, therefore, to add the two-way travel time of the water bottom to the time of the generating primary event and look for correspondence between the ‘phantom’ horizon thus created and seismic reflections. Such techniques for multiple identification are subjective and qualitative.
Li et al. (“Li”) suggest producing separate images from the up-going and down-going parts of a wavefield separately using pre-stack depth migration. (“Combining dual-sensor data with pre-stack depth migration—Imaging the ghost and primary reflection at Teal South,” SEG Technical Program Expanded Abstracts, 1190-1193 (1999)) Then, they propose combining the four separate images thus produced (after “calibration of the four separate migration images”), with the objective of suppressing multiples in order to improve the primary image. Li's approach and that of the present invention combine hydrophone and geophone data post-stack, which differs from conventional approaches.
In Li's method, four seismic images are produced from: (1) up-going wavefield based on vertical geophone data; (2) down-going wavefield based on vertical geophone data; (3) up-going wavefield based on hydrophone data; and, (4) down-going wavefield based on the hydrophone data. Li uses conventional imaging for two of the four images they produce (the up-going); the other two (down-going) are obtained using “mirror” imaging, i.e., imaging from fictitious receiver elevations.) What the authors call “incorrectly imaged energy” remains, i.e., down-going energy that appears below the correct image of the reflector on the up-going image and up-going energy that appears above the correct image of the reflector on the down-going image. This energy is partly attenuated using a stack of the four separate images.
Li's method accounts only for the first order water column reverberations. The authors note, “imaging the reverberations that undergo more than a single pass through the water column . . . require[s] more complicated treatment”. Furthermore, in separating the “primaries” and the “incorrectly imaged energy”, the method relies on kinematic (i.e., moveout) differences between the two (not amplitudes). Thus the effectiveness of Li's method depends greatly on water depth. The authors note this in their paper: “How much we can benefit from using down-going image of the ghost depends on the water depth.” Li's method is thus a post-stack combination of 2C OBS data that requires production of four images of the subsurface: two of the up-going wavefield and two of the down-going wavefield.
For the above-described reasons, an improved method of identifying and correcting OBS data for multiples is needed. The present invention satisfies this need.